Hydrogen can readily be produced by well-known processes such as the partial oxidation of a hydrocarbon with air or oxygen and the steam reforming of hydrocarbons or alcohols with steam. Historically, the petrochemical industry has been the major producer of hydrogen, producing large volumes of hydrogen for use in various on-site industrial processes. Not surprisingly, such production has failed to produce an infrastructure for the widespread production and distribution of hydrogen.
More recently, advances in fuel cell technology have prompted the development of technologies for smaller-scale production of hydrogen suitable for use in mobile and stationary fuel cell systems. As is well known, fuel cells generate electricity from chemical oxidation-reduction reactions and can provide several advantages over other forms of power generation. For example, fuel cells provide cleaner vehicle emissions, quieter operation and can have higher efficiencies than other power generation systems. Such advantages have lead to increasing demand for fuel cells and hydrogen production systems appropriate for fueling them.
Typically, a fuel processor or reformer is used to convert a fuel source, such as a hydrocarbon(s) and/or alcohol(s) to a hydrogen-rich reformate. However, the reforming of such fuels generally produces a hydrogen-rich reformate that contains impurities such as carbon monoxide, carbon dioxide, and potentially sulfur and nitrogen-containing compounds as well. Carbon monoxide is frequently present in such reformate compositions at concentrations that will poison fuel cell catalysts. In the case of polymer electrolyte membrane (PEM) fuel cells, levels of carbon monoxide exceeding 100 ppm cannot be tolerated by the cell's catalyst and levels as low as 5 ppm can have a significant adverse effect on fuel cell performance. As a result, the reduction and/or removal of carbon monoxide from a hydrogen-rich reformate intended for PEM fuel cell applications is of particular concern.
It is known that the level of carbon monoxide in a reformate composition can be reduced by utilizing a water-gas shift reaction. To achieve this, water, e.g. steam, is added to a flow of reformate to lower its temperature and to increase the steam-to-carbon ratio. During the reaction, carbon monoxide and water are catalytically converted to carbon dioxide and hydrogen according to the equationCO+H2O→CO2+H2.Lower temperatures and higher steam-to-carbon ratios favor this shift reaction. However, while a water gas shift reaction is valuable for converting large amounts of carbon monoxide to carbon dioxide, the reaction is not well suited for removing trace amounts of carbon monoxide. Because reformate compositions subjected to shift reactions can still contain detrimental amounts of carbon monoxide, it is generally necessary to further reduce the level of carbon monoxide through other means.
The carbon monoxide content of the reformate can be further reduced through a preferential oxidation reaction. The preferential oxidation of carbon monoxide is described by Choi et al., in a paper entitled, “Kinetics, Simulation And Insights For CO Selective Oxidation In Fuel Cell Applications,” Journal of Power Sources, vol. 129, pp. 246-254 (2004), and in U.S. Pat. No. 5,271,916 to Vanderbourgh, each of which is incorporated herein by reference. Generally, preferential oxidation reactors may be either (1) adiabatic, wherein the temperature of the catalyst is allowed to rise due to the exothermic nature of the reaction(s), or (2) isothermal, wherein the temperature of the catalyst is maintained substantially constant by removing the heat generated by the reaction(s). Adiabatic systems typically include a number of sequential stages that reduce the carbon monoxide content in a stepwise fashion so as to avoid excessively high temperatures that might otherwise be produced in a single stage reactor.
A preferential oxidation reactor contains an oxidation catalyst for oxidizing carbon monoxide according to the equationCO+½O2→CO2.Because hydrogen is present in the hydrogen-rich reformate, there are two competing reactions that can also occur, namely, the oxidation of hydrogenH2+½O2→H2O,and a reverse water-gas shift reactionH2+CO2→CO+H2O.
As shown by these equations, the carbon monoxide oxidation reaction and the hydrogen oxidation reaction directly compete for available oxygen. While both reactions are exothermic, the oxidation of carbon monoxide is slightly more so. As a result, lower catalyst or reaction temperatures tend to favor the oxidation of carbon monoxide over the oxidation of hydrogen. Moreover, the use of excessive amounts of oxygen should also be avoided to inhibit the oxidation of hydrogen. The reverse water-gas shift reaction is an equilibrium reaction that tends to occur when there are low levels of available oxygen. In addition, the reverse water-gas shift reaction favored at low carbon monoxide concentrations and higher temperatures. Because both the carbon monoxide and hydrogen oxidation reactions are exothermic, favorable conditions for the reverse water-gas shift reaction tend to develop as the oxidation reaction(s) progress.
In summary, lower reaction temperatures tend to favor the oxidation of carbon monoxide while higher reaction temperatures favor both the hydrogen oxidation and the reverse water gas shift reactions. Moreover, the oxidation catalyst can be deactivated and/or damaged if excessively high reaction temperatures are allowed to develop with the reactor. As a result, improved temperature control is essential to the efficient oxidation of carbon monoxide. Therefore, it is desirable to provide an improved reactor, apparatus and method for reducing the carbon monoxide concentration in a hydrogen-rich reformate.